1. Technical Field
The invention relates to thermo-optic waveguide structures in which an optical characteristic of the structure changes in dependence upon a change in temperature, and to devices employing such a waveguide structure. The invention also relates to such waveguide structures having means for monitoring variations in temperature of part of the waveguide structure.
2. Background Art
This specification refers to several published articles, For convenience, the articles are cited in full in a numbered list at the end of the description and cited by number in the specification itself. The contents of these articles are incorporated herein by reference and the reader is directed to them for reference.
In the context of this patent specification, the term “optical radiation” embraces electromagnetic waves having wavelengths in the infrared, visible and ultraviolet ranges.
The terms “finite” and “infinite” as used herein are used by persons skilled in this art to distinguish between waveguides having “finite” widths in which the actual width is significant to the performance of the waveguide and the physics governing its operation and so-called “infinite” waveguides where the width is so great that it has no significant effect upon the performance and physics or operation.
It is well known that refractive index changes can be used to manipulate the propagation of light. Depending on the material, refractive index changes can be induced thermally, electro-optically, or otherwise. The thermo-optic (TO) effect is present in a material when a change in its temperature causes a change in its refractive index. In known optical devices, the usual configuration in which the TO effect is exploited comprises a thin film heater deposited on top of a TO material which is located in the propagation path of the light. The propagation of light can then be affected by injecting a current through the thin film heater which will generate heat via ohmic losses. The TO effect has been used in the prior art for many applications, such as to tune Bragg gratings, switch, multiplex, demultiplex, attenuate or modulate an optical signal
It is known to construct thermally activated variable optical attenuators (VOA's) from a straight waveguide section where the propagating mode is thermally induced into cut-off, or from a Mach-Zehnder Interferometer (MZI) structure where the insertion phase of one arm is thermally changed to create destructive interference. It is also known that thermally-activated couplers, MZI's and Gragg gratings can be assembled to enable switching and tunable filtering functions.
Much effort has been devoted to improving such devices. In an article entitled “Variable Optical Attenuator Based upon a Cut Off Modulator with Tapered Waveguides in Polymers”, Journal of Lightwave Technology Vol. 17, No. 12, December 1999, pp. 2556–2561, Sang-Shin Lee et al. revealed that, in a cut-off modulator, heating of the dielectric waveguide via a thin film heater reduced the lateral confinement of the structure. The dynamic range was more than 20 dB with an electrical power consumption of 160 mW. The optical response time was faster than 1.5 ms. In U.S. Pat. No. 6,507,681, Bischel et al. disclosed a similar structure and claimed that, by heating sufficiently, they could create an anti-waveguiding region which improved the extinction ratio of the device. In their U.S. patent application Ser. No. 2002/0018636, Bischel et al. disclosed a VOA in which a heater above a waveguide created an asymmetric refractive index profile along the vertical axis extending between the thermal source and the heat sink. The optical energy propagating along the core was transversely deflected in the region away from the thermal source. They claim that the device is able to achieve 30 dB attenuation with 15 mW of power consumption with rise and fall times of at least 270 μs and 1260 μs respectively.
Other examples of known VOA's based on the MZI structure include IEEE Photonics Technology Letters, vol. 5, No. 7, July 1993 pp. 782–784, “Polymer Waveguide Thermooptic Switch with Low Electric Power Consumption at 1.3 μm” by Yasuhiro Hida et al. The authors claim that the device achieves a π-phase shift with 4.8 mW of power consumption and rise and fall times of 9 ms.
U.S. Pat. No. 5,173,956 discloses TO switching of an optical waveguide by means of a Schottky diode created on top of a ridge waveguide fabricated from GaAs. A ground plane is deposited under the device. The Schottky diode is forward biased and a current flows between the electrode and the ground plane causing heating of the semiconductor. Heat is localized to the Schottky diode region. It is claimed that switching times are 0.3 microsecond while power consumption varies between 100 mW and 800 mW depending on the desired mode of operation of the device. Internal heating is caused by a current flowing between the electrode and the ground plane, with heating localized within the region of current flow. However, only a fraction of the heated region overlaps well with the waveguide to influence the propagation of light, resulting in less than optimal energy efficiency for the device.
Generally, despite such efforts to improve response times for a given power, these known devices are not entirely satisfactory, primarily because the heat source is located a distance away from the optical waveguide core. Consequently, heat must diffuse from the source into the waveguide core in order to affect the light confined therein, which is a time consuming process. Furthermore, heat diffuses away from the source in many directions: while some of the heat is directed towards the waveguide, a significant amount is lost and unused.
It is known to add trenches each side of the waveguide to limit lateral heat diffusion, but this is at the expense of increased fabrication complexity and slower switching times. Also, some of the prior art devices are sensitive to the ambient temperature and require temperature control. Finally, device size and manufacturing costs are other problems with some of the prior art devices.
Thus, a disadvantage of these known devices generally is that they suffer from one or more of the following limitations: high power consumption, low energy efficiency, complex temperature control requirements, large device size, complex fabrication requirements, inaccurate thermal sensing, and poor optical performance.